Re: Preview of new stegosaur plate paper

[Mike Taylor <mike@miketaylor.org.uk>]

> Date: Wed, 18 May 2005 07:28:07 -0700 (PDT)
> From: don ohmes <d_ohmes@yahoo.com>
> 
> If plate size scales with body size in accordance with
> thermoregulatory function, I personally find that 
> compelling evidence that thermoregulatory selection is
> the primary driver of plate size.

Not necessarily; it could easily happen that the allometric
exponential for thermoregulatory function (=~ metabolic rate, I guess)
is the same as that for some other variable.  It would be a clue, but
far from conclusive.

> Engineering and mass allocation considerations indicate that the
> relative size of passive defense structures must scale negatively
> with body size.

How so?

> Intuitively, it seems to me that the relative size of display
> structures might scale negatively with body size. Does anyone have
> enough data from extant animals to comment on this?

I don't have references to my fingertips, but I do recall that,
surprisingly, this is often incorrect.  For example, the ludicrously
big antlers of the recent deer _Megaloceros_ ("Irish Elk") scale
_positively_ with body size, so that large specimens have
proportionally larger antlers than smaller individuals.

This is, IIRC, discussed in one of Chris McGowan's books, quite
possibly _A Practical Guide to Vertebrate Mechanics_ as this has a
_Megaloceros_ skeleton on the front cover:
        http://www.miketaylor.org.uk/dino/books/index.html#gvm

To bring the discussion back to sauropods (hey, that's while we're all
here, right?) it appears that many of the very large sauropods seem to
have long necks even by sauropod standards.  Among these I cite
_Supersaurus_, _Brachiosaurus_ and (even more so) _Sauroposeidon_.  By
contrast, smaller sauropods such as _Saltasaurus_ seems to have
proportionally short necks.  Clearly this is only anecdotal, and
someone ought to take a stab at analysing this statistically.  But if
this positive allometry is real, then it could be read as an
indication in favour of the idea that those big ol' necks were nothing
more than penis substitutes (i.e. sexual display organs).

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